Electronic underfrequency relay

• US 3,993,984 A
• Filed: 04/08/1975
• Issued: 11/23/1976
• Est. Priority Date: 04/08/1975
• Status: Expired due to Term

• Current Assignee: Power Management Corporation
• Original Assignee: Power Management Corporation
• Inventors: Penrod, John K.
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): Tokar, Michael J.
• Application Number:
  US05/566,094
• Time in Patent Office: 595 Days
• Field of Search: 324/79 D, 324/186, 324/187, 324/181, 324/78 D, 324/78 Q, 324/78 Z, 307/233 R, 307/234, 328/133, 328/134, 340/253 Y, 340/248 A, 235/92 FQ, 235/92 TF
• US Class Current: 340/658
• CPC Class Codes: G01R 23/15 Indicating that frequency of pulses is either above or below a predetermined value or within or outside a predetermined range of values, by making use of non-linear or digital elements (indicating that pulse width is above or below a certain limit)
• CPC Class Codes: H02H 3/46 responsive to frequency deviations

Abstract

A method and apparatus for detecting an underfrequency condition of an alternating current power line is disclosed wherein the frequency of the power line is sensed, an underfrequency signal is developed whenever the power line frequency decreases below a predetermined frequency, a known frequency source is thereafter substituted for the alternating current power line, and the frequency of this source is checked by the same circuitry to produce a second output signal, and an underfrequency signal is generated in response to both first and second output signals.

Claims

1. A method of detecting and indicating an underfrequency condition of an alternating current power line including the steps of

sensing the frequency of the alternating current power line by circuit means which measures the time interval between adjacent cycles of said line frequency, and providing a first signal whenever the frequency of the power line decreases below a predetermined frequency;

generating a known frequency signal and substituting said known frequency signal for the alternating current line frequency in response to said first signal, measuring the time interval between adjacent cycles of the known frequency signal by said curcuit means, and providing a second signal if the frequency of said known frequency signal is detected to be within predetermined limits; and

indicating a power line underfrequency condition only in response to both said first and second signals.

2. The method of detecting an underfrequency condition on an alternating current power line including the steps of

generating clock pulses at a fixed frequency substantially higher than said alternating current line frequency,

counting said clock pulses occurring during the interval between adjacent cycles of the line frequency by a counting circuit means,

generating a first output signal upon the detection of a number of clock pulses in said interval greater than a predetermined number,

generating a known frequency signal and substituting said signal for said alternating line current frequency, in response to said first output signal, as an input to said counting circuit means,

counting the number of clock pulses occurring during the interval between adjacent cycles of the known frequency signal by said counting circuit means,

generating a second output signal upon detection of not more than a predetermined number of clock pulses within said interval between adjacent cycles of the known frequency signal, and

generating a power line underfrequency signal in response to both said first and second output signals.

3. Apparatus for detecting an underfrequency condition on an alternating current power line including

means for generating clock pulses at a fixed frequency substantially higher than said alternating current line frequency,

counter means for counting the number of clock pulses occurring during the interval between adjacent cycles of said line frequency,

means for generating a first output signal whenever the number of clock pulses in said interval is greater than an adjustable predetermined number,

means for generating a known frequency signal,

means responsive to said first output signal for substituting said known frequency signal for said alternating current power line frequency as an input to said counter means,

means for generating a second output signal in response to the output of said counter containing not more than a predetermined number of clock pulses within the interval between adjacent cycles of the known frequency signal, and

means for generating a power line underfrequency signal only when both said first and second output signals have been generated.

4. The apparatus of claim 3 including means for inhibiting an underfrequency signal from being generated upon detection of a power line frequency above a predetermined minimum frequency.